1. Field of the Invention
The present invention relates generally to heat exchange systems, and in particular, to a gas-to-water heat recovery system which utilizes an array of heat pipes for collecting heat from a stream of heated gas and transferring the heat into a volume of water for the production of steam.
2. Description of the Prior Art
Heat recovery from industrial waste gas sources presents an ever increasing opportunity for economical operation of thermal systems. The economic advantage from any form of heat recovery depends upon the availability and cost of fuels. Obviously, savings from heat recovery increase as fuel costs rise. As the cost of energy constantly increases, various systems and methods are being devised to recover and transfer thermal energy which would otherwise be lost.
Conventional heat exchange apparatus operates in several heat recovery modes including air to gas, gas to water, and gas to organic fluids. The selection of the mode of heat recovery depends upon the characteristics of the application, the processes used by the particular industrial facility, and the economic need for a given service. For example, steam can be generated at low pressure for heating or absorption air conditioning applications, at medium pressures for processing, or at higher pressures with or without superheat for electrical power generation.
The recovery of heat energy by the generation of either high pressure or low pessure steam is probably the most common means of fuel and energy conservation because steam carries tremendous heat energy per unit weight, consisting of sensible and latent heat. Various types of heat recovery boilers are available for the recovery of heat energy by the generation of either high pressure or low pressure steam. Examples of conventional heat recovery boilers include units of straight tube banks attached to fixed or floating headers and units of serpentine (return bend) elements. The circular coil type and the horizontal serpentine element type require forced recirculation. Vertical tube units may operate in either forced or natural circulation modes. Larger low pressure heat recovery applications usually employ the natural circulation system, commonly of the two drum variety.
In the operation of conventional waste heat boilers, the rate of heat transfer from waste gases to the boiler water depends upon the temperature and specific heat of the gases, the velocity and direction of the gases over the heat absorbing surfaces of the boiler, and the cleanliness of the surfaces. For proper heat transfer from the waste gases to the boiler water there must be sufficient stack or an induced draft fan to overcome the draft losses due to the required flow of the gases over the heat absorbing surfaces with an allowance for fouling of these surfaces. Compared with direct firing arrangements, the gas temperatures are generally lower and consequently the radiation component in the heat transfer mechanism is also lower. Therefore, the tendency with waste heat boilers is to design for higher gas velocity over the tubes in order to increase the convection component of heat transfer. However, a significant number of industrial processes generate a substantial amount of heated waste gas which is only available for recovery of thermal energy at relatively low flow velocities. Consequently, there exist a number of industrial processes in which recovery of waste heat by conventional heat exchangers is relatively inefficient because of the low flow velocities involved. In view of the constantly increasing cost of energy, there is a continuing need for new and improved systems for recovering waste heat which operate effectively at relatively low flow velocities.
The use of heat pipes in combination with a steam boiler offers several advantages over conventional heat exchange arrangements. For example, the transducer characteristic of the heat pipe permits collection of heat from a diffused source such as low velocity waste gas and transfer of the heat into a concentrated thermal sink such as a volume of water. Sealing a heat pipe through a single or double wall header plate provides complete isolation of one fluid stream from the other. Because of the single point connection, both the evaporator and condenser ends of the heat pipe extend freely thereby minimizing stress problems due to thermal expansion and contraction. Furthermore, the outside of the heat pipe is available in both fluid streams for cleaning, for extended surface fin structure, or for special surface preparation to enhance heat transfer.
In some conventional heat exchanger arrangements in which heat pipes are utilized for transferring waste heat from a heated gas stream to a steam boiler, the heat transfer efficiency has been limited by losses associated with sealing the heat pipes through the header plate in the wall of the boiler tank. Unacceptable levels of heat transfer have occurred through the walls of the heat pipe at the heat pipe/header plate interface because of the large interface area involved. A relatively large heat transfer interface area between the heat pipe and the header plate has resulted in part because the header plate must be relatively thick as compared with the thickness of the steam boiler walls in order to meet the strength and pressure requirements and other provisions of the ASI code for boiler construction. It is, therefore, the principal object of the present invention to provide a heat pipe support arrangement which provides mechanical support for a heat pipe, provides a fluid-tight seal for the interior of a steam boiler, and which minimizes heat transfer from the heat pipe through the header plate.